Field effect transistor

ABSTRACT

It is an object to provide a low-cost oxide semiconductor material which is excellent in controllability of the carrier concentration and stability, and to provide a field effect transistor including the oxide semiconductor material. An oxide including indium, silicon, and zinc is used as the oxide semiconductor material. Here, the content of silicon in the oxide semiconductor film is greater than or equal to 4 mol % and less than or equal to 8 mol %. The field effect transistor including such an In—Si—Zn—O film can withstand heat treatment at a high temperature and is effective against −BT stress.

TECHNICAL FIELD

The present invention relates to a field effect transistor including anoxide semiconductor.

BACKGROUND ART

In recent years, an oxide semiconductor has attracted attention as anovel semiconductor material having high mobility, which is an advantageof polysilicon, and a uniform element characteristic, which is anadvantage of amorphous silicon.

In Patent Document 1, a field effect transistor which includes, as anoxide semiconductor, an oxide including indium (In), zinc (Zn), andgallium (Ga) (a material having an In—Ga—Zn—O composition) has beenproposed.

[Reference]

-   [Patent Document 1] Japanese Published Patent Application No.    2006-173580

DISCLOSURE OF INVENTION

However, a material having an In—Ga—Zn—O composition includes anexpensive raw material and thus has a problem of high cost.

In view of the above problem, it is an object to provide a low-costoxide semiconductor material that is excellent in controllability ofcarrier concentration and stability, and to provide a field effecttransistor including the oxide semiconductor material.

An oxide including indium (In), silicon (Si), and zinc (Zn) (a materialhaving an In—Si—Zn—O composition) is used as an oxide semiconductormaterial. Here, the content of Si in the oxide semiconductor film isgreater than or equal to 4 mol % and less than or equal to 8 mol %.

An embodiment of the present invention is a field effect transistorincluding a gate electrode, a gate insulating film, an oxidesemiconductor film, a source electrode, and a drain electrode. In thefield effect transistor, the oxide semiconductor film is an oxideincluding indium, silicon, and zinc, and the content of silicon in theoxide semiconductor film is greater than or equal to 4 mol % and lessthan or equal to 8 mol %.

With the use of a material having an In—Si—Zn—O composition, a fieldeffect transistor having stable characteristics can be provided at lowcost.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIG. 1 is a cross-sectional schematic view of a field effect transistorincluding an oxide semiconductor;

FIG. 2 is a graph showing results of comparison between the Si contentin an In—Si—Zn—O target and the Si content in an In—Si—Zn—O film;

FIG. 3 is a graph showing results of measuring an In—Si—Zn—O film by anX-ray diffraction (XRD) analysis method;

FIG. 4 is a graph showing dependence of Hall effect mobility of anIn—Si—Zn—O film on the Si content in the film;

FIG. 5 is a graph showing initial characteristics of a field effecttransistor ([1] Si=0 [mol %], and heat treatment condition is at 350° C.in an N₂ atmosphere);

FIG. 6 is a graph showing initial characteristics of a field effecttransistor ([1] Si=0 [mol %], and heat treatment condition is at 450° C.in an N₂ atmosphere);

FIG. 7 is a graph showing initial characteristics of a field effecttransistor ([1]Si=0 [mol %], and heat treatment condition is at 600° C.in an N₂ atmosphere and then changed to at 450° C. in dry air);

FIG. 8 is a graph showing initial characteristics of a field effecttransistor ([2] Si=2 [mol %], and heat treatment condition is at 350° C.in an N₂ atmosphere);

FIG. 9 is a graph showing initial characteristics of a field effecttransistor ([2] Si=2 [mol %], and heat treatment condition is at 450° C.in an N₂ atmosphere);

FIG. 10 is a graph showing initial characteristics of a field effecttransistor ([2] Si=2 [mol %], and heat treatment condition is at 600° C.in an N₂ atmosphere and then changed to at 450° C. in dry air);

FIG. 11 is a graph showing initial characteristics of a field effecttransistor ([3] Si=4 [mol %], and heat treatment condition is at 350° C.in an N₂ atmosphere);

FIG. 12 is a graph showing initial characteristics of a field effecttransistor ([3] Si=4 [mol %], and heat treatment condition is at 450° C.in an N₂ atmosphere);

FIG. 13 is a graph showing initial characteristics of a field effecttransistor ([3] Si=4 [mol %], and heat treatment condition is at 600° C.in an N₂ atmosphere and then changed to at 450° C. in dry air);

FIG. 14 is a graph showing initial characteristics of a field effecttransistor ([4] Si=8 [mol %], and heat treatment condition is at 350° C.in an N₂ atmosphere);

FIG. 15 is a graph showing initial characteristics of a field effecttransistor ([4] Si=8 [mol %], and heat treatment condition is at 450° C.in an N₂ atmosphere);

FIG. 16 is a graph showing initial characteristics of a field effecttransistor ([4]Si=8 [mol %], and heat treatment condition is at 600° C.in an N₂ atmosphere and then changed to at 450° C. in dry air);

FIG. 17 is a graph showing results of a +BT test performed on a fieldeffect transistor ([1] Si=0 [mol %]);

FIG. 18 is a graph showing results of a −BT test performed on the fieldeffect transistor ([1] Si=0 [mol %]);

FIG. 19 is a graph showing results of a +BT test performed on a fieldeffect transistor ([2] Si=2 [mol %]);

FIG. 20 is a graph showing results of a −BT test performed on the fieldeffect transistor ([2] Si=2 [mol %]);

FIG. 21 is a graph showing results of a +BT test performed on a fieldeffect transistor ([3] Si=4 [mol %]);

FIG. 22 is a graph showing results of a −BT test performed on the fieldeffect transistor ([3] Si=4 [mol %]);

FIG. 23 is a graph showing results of a +BT test performed on a fieldeffect transistor ([4] Si=8 [mol %]);

FIG. 24 is a graph showing results of a −BT test performed on the fieldeffect transistor ([4] Si=8 [mol %]);

FIGS. 25A to 25D illustrate a manufacturing process of the field effecttransistor illustrated in FIG. 1; and

FIGS. 26A to 26D illustrate the manufacturing process of the fieldeffect transistor illustrated in FIG. 1.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the invention disclosed in this specification will bedescribed below with reference to the drawings. Note that the inventionis not limited to the following description, and it is easily understoodby those skilled in the art that modes and details of the invention canbe modified in various ways without departing from the spirit and thescope of the invention. Therefore, the invention should not be construedas being limited to the following description of the embodiments.

(Embodiment 1)

FIG. 1 is a cross-sectional schematic view of a field effect transistorincluding an oxide semiconductor. The field effect transistor includes asubstrate 10, a base insulating film 20, a gate electrode 30, a gateinsulating film 40, an oxide semiconductor film 50, and a metal film 60.Note that the oxide semiconductor film 50 is an In—Si—Zn—O film.

The field effect transistor illustrated in FIG. 1 has a channel-etchedbottom-gate structure. Note that the structure of the field effecttransistor is not limited thereto, and can be a desired top-gate orbottom-gate structure.

It is appropriate that a glass substrate is used as the substrate 10. Inthe case where heat treatment is performed later at a high temperature,a glass substrate whose strain point is 730° C. or higher is preferablyused. In addition, from the viewpoint of heat resistance, a glasssubstrate which includes more barium oxide (BaO) than boron oxide (B₂O₃)is preferably used.

A substrate formed using an insulator such as a ceramic substrate, aquartz glass substrate, a quartz substrate, or a sapphire substrate mayalso be used as the substrate 10 instead of the glass substrate.Alternatively, a crystallized glass substrate or the like can be used asthe substrate 10.

The base insulating film 20 has a function of preventing diffusion of animpurity element from the substrate 10. Note that the base insulatingfilm 20 can be formed using one or more films selected from a siliconoxide film, a silicon nitride film, a silicon oxynitride film, and asilicon nitride oxide film.

Note that in the case where an insulating substrate is used as thesubstrate 10, the base insulating film 20 does not need to be provided.That is, the gate electrode 30 may be formed over the substrate 10having an insulating surface.

A metal conductive film can be used as the gate electrode 30. As amaterial of the metal conductive film, an element selected from aluminum(Al), chromium (Cr), copper (Cu), tantalum (Ta), titanium (Ti),molybdenum (Mo), and tungsten (W); an alloy including any of theseelements as a component; or the like can be used. For example, athree-layer structure of a titanium film, an aluminum film, and atitanium film; a three-layer structure of a molybdenum film, an aluminumfilm, and a molybdenum film; or the like can be employed. Note that themetal conductive film is not limited to a three-layer structure, and mayhave a single-layer structure, a two-layer structure, or a stackedstructure of four or more layers.

The gate insulating film 40 is in contact with the oxide semiconductorfilm 50 and thus is preferably a dense film with high withstand voltage.Therefore, it is particularly preferable that the gate insulating film40 be formed by a high-density plasma CVD method using a microwave (2.45GHz). This is for reduction of plasma damage in the formation of thegate insulating film 40. As a result, defects generated in the gateinsulating film 40 can be reduced, and the condition of an interfacewith the oxide semiconductor film 50 formed later can be favorable. Ifthe condition of the interface between the oxide semiconductor film 50and the gate insulating film 40 is unfavorable, a dangling bondgenerated when a bond between an impurity and a main component of theoxide semiconductor is cut causes a shift in threshold voltage in abias-temperature (BT) test, which is a typical test for evaluatingreliability of a field effect transistor.

It is preferable that the gate insulating film 40 include impuritiessuch as moisture and hydrogen as little as possible. In addition, thegate insulating film 40 can be formed using a film of silicon oxide,silicon nitride, silicon oxynitride, silicon nitride oxide, aluminumoxide, aluminum nitride, aluminum oxynitride, aluminum nitride oxide,hafnium oxide, or the like.

The oxide semiconductor film 50 is the In—Si—Zn—O film as describedabove, and the Si content in the film is greater than or equal to 4 mol% and less than or equal to 8 mol %.

Impurities such as hydrogen, moisture, a hydroxyl group, and hydroxide(also referred to as a hydrogen compound) which are thought to act asdonors are intentionally removed from the oxide semiconductor film 50,and then oxygen which is simultaneously reduced in the step of removingthese impurities is supplied to the oxide semiconductor film 50.Accordingly, the oxide semiconductor film 50 is purified and becomeselectrically i-type (intrinsic). The purpose of this treatment is tosuppress fluctuation in electric characteristics of the field effecttransistor.

The smaller the concentration of hydrogen in the oxide semiconductorfilm 50 is, the closer to the i-type the oxide semiconductor film 50 is.Therefore, the concentration of hydrogen included in the oxidesemiconductor film 50 is preferably 5×10¹⁹ atoms/cm³ or lower, morepreferably 5×10¹⁸ atoms/cm³ or lower, still more preferably 5×10¹⁷atoms/cm³ or lower, further more preferably lower than 5×10¹⁶ atoms/cm³.The concentration of hydrogen can be measured by secondary ion massspectrometry (SIMS).

In an oxide semiconductor, which is a wide bandgap semiconductor, thedensity of minority carriers is low and the minority carriers are lesslikely to be induced. Thus, it can be said that, in the field effecttransistor including the oxide semiconductor film 50, tunnel current isdifficult to be generated; consequently, off-state current is difficultto flow.

In addition, impact ionization and avalanche breakdown are less likelyto occur in the field effect transistor including the oxidesemiconductor film 50 which is formed using a wide bandgapsemiconductor. Therefore, it can be said that the field effecttransistor including the oxide semiconductor film 50 has resistance tohot carrier deterioration. This is because hot carrier deterioration ismainly caused by increase in the number of carriers due to avalanchebreakdown and by injection of the carriers accelerated to high speed tothe gate insulating film.

The metal film 60 is used as a source electrode or a drain electrode.For the metal film 60, a metal material such as aluminum (Al), chromium(Cr), copper (Cu), tantalum (Ta), titanium (Ti), molybdenum (Mo), ortungsten (W); or an alloy material including any of these metalmaterials as a component can be used. In addition, the metal film 60 mayhave a structure in which a film of refractory metal such as chromium(Cr), tantalum (Ta), titanium (Ti), molybdenum (Mo), or tungsten (W) isstacked on one side or both sides of a metal film of aluminum (Al),copper (Cu), or the like. Note that an aluminum material to which anelement that prevents generation of hillocks or whiskers in an aluminumfilm, such as silicon (Si), titanium (Ti), tantalum (Ta), tungsten (W),molybdenum (Mo), chromium (Cr), neodymium (Nd), scandium (Sc), oryttrium (Y), is added is used, whereby the metal film 60 with high heatresistance can be obtained.

(Embodiment 2)

A manufacturing process of a field effect transistor having thestructure illustrated in FIG. 1 will be described with reference toFIGS. 25A to 25D and FIGS. 26A to 26D.

As illustrated in FIG. 25A, the base insulating film 20 is formed overthe substrate 10. As illustrated in FIG. 25B, a conductive film 35 isformed over the base insulating film 20. As illustrated in FIG. 25C, thegate electrode 30 is formed in a first photolithography process. Asillustrated in FIG. 25D, the gate insulating film 40 is formed over thegate electrode 30. As illustrated in FIG. 26A, an oxide semiconductorfilm 55 is formed over the gate insulating film 40. As illustrated inFIG. 26B, the oxide semiconductor film 55 is etched so that the oxidesemiconductor film 50 is formed. As illustrated in FIG. 26C, a metalfilm 65 is formed over the oxide semiconductor film 50. As illustratedin FIG. 26D, the metal film 65 is etched so that the metal film 60 isformed. The field effect transistor illustrated in FIG. 1 is obtainedthrough the above steps.

A supplementary explanation on some of the steps is given below.

In the formation step of the gate electrode 30 illustrated in FIG. 25C,a resist mask used in the first photolithography process may be formedby an inkjet method. When the resist mask is formed by an inkjet method,a photomask is not used; therefore, manufacturing cost can be reduced.

In the formation step of the gate insulating film 40 illustrated in FIG.25D, the gate insulating film 40 is formed by a sputtering method, forexample. It is preferable that, as pretreatment performed before thefilm formation, the substrate 10 provided with the gate electrode 30 bepreheated in a preheating chamber of a sputtering apparatus so thatimpurities such as hydrogen and moisture adsorbed to the substrate 10may be removed and eliminated. The purpose of this preheating is toprevent the impurities such as hydrogen and moisture from being includedin the gate insulating film 40 and the oxide semiconductor film 50 whichare formed later as much as possible. In addition, the substrate 10 overwhich films up to the gate insulating film 40 are formed may bepreheated.

The appropriate temperature of the preheating is higher than or equal to100° C. and lower than or equal to 400° C. A temperature of higher thanor equal to 150° C. and lower than or equal to 300° C. is morepreferable. In addition, a cryopump is preferably used as an evacuationunit in the preheating chamber.

In the formation step of the oxide semiconductor film 55 illustrated inFIG. 26A, the oxide semiconductor film 55 is formed by a sputteringmethod.

Before the oxide semiconductor film 55 is formed, the substrate 10 isheld in a treatment chamber in a reduced pressure state, and thesubstrate 10 is heated to a temperature of higher than or equal to roomtemperature and lower than 400° C. Then, while a sputtering gas fromwhich hydrogen and moisture are removed is introduced in the state wheremoisture remaining in the treatment chamber is removed, voltage isapplied between the substrate 10 and a target, so that the oxidesemiconductor film 55 is formed over the substrate 10.

It is appropriate that an entrapment vacuum pump is used as theevacuation unit for removing moisture remaining in the treatmentchamber. For example, a cryopump, an ion pump, and a titaniumsublimation pump can be given. Alternatively, a turbo pump provided witha cold trap can be used as the evacuation unit. From the treatmentchamber, a compound including a hydrogen atom, such as water (H₂O), orthe like (more preferably, also a compound including a carbon atom) iseliminated; thus, the concentration of impurities included in the oxidesemiconductor film 55 which is formed in the treatment chamber can bereduced. Further, film formation by sputtering is performed whilemoisture remaining in the treatment chamber is removed with a cryopump;thus, the temperature of the substrate 10 at the time of forming theoxide semiconductor film 55 can be set higher than or equal to roomtemperature and lower than 400° C.

Note that before the oxide semiconductor film 55 is formed by asputtering method, dust attached to a surface of the gate insulatingfilm 40 is preferably removed by reverse sputtering. The reversesputtering refers to a method in which a substrate surface is cleanedwith reactive plasma generated by voltage application to the substrateside using an RF power source without voltage application to a targetside. Note that the reverse sputtering is performed in an argonatmosphere. Alternatively, nitrogen, helium, oxygen, or the like may beused instead of argon.

After the etching step of the oxide semiconductor film 55 illustrated inFIG. 26B, heat treatment for dehydration or dehydrogenation of the oxidesemiconductor film 50 is performed. It is appropriate that the heattreatment for dehydration or dehydrogenation is performed at atemperature of higher than or equal to 350° C. and lower than or equalto 750° C.

For example, the heat treatment for dehydration or dehydrogenation isperformed in a nitrogen atmosphere by putting the substrate 10 providedwith the oxide semiconductor film 50 in an electric furnace which is akind of heat treatment apparatus. After that, a high-purity oxygen gas,a high-purity dinitrogen monoxide (N₂O) gas, or ultra-dry air (a gas inwhich nitrogen and oxygen are mixed at a ratio of 4:1 and which has adew point of −40° C. or lower, preferably −60° C. or lower) isintroduced into the same furnace and cooling is performed. It ispreferable that water, hydrogen, and the like be not included in theoxygen gas or the N₂O gas. In addition, it is appropriate that thepurity of an oxygen gas or an N₂O gas is 6N (99.9999%) or higher,preferably 7N (99.99999%) or higher (i.e., the concentration ofimpurities in the oxygen gas or the N₂O gas is 1 ppm or lower, morepreferably 0.1 ppm or lower).

Note that the heat treatment apparatus is not limited to the electricfurnace; for example, a rapid thermal anneal (RTA) apparatus such as agas rapid thermal anneal (GRTA) apparatus or a lamp rapid thermal anneal(LRTA) apparatus can be used.

Note that the heat treatment for dehydration or dehydrogenation may beperformed after the formation step of the oxide semiconductor filmillustrated in FIG. 26A.

Example 1

<In—Si—Zn—O film>

Four kinds of In—Si—Zn—O films were formed using targets havingdifferent compositions, and characteristics of the In—Si—Zn—O films werecompared. The compositions of the targets are the following [1] to [4]:

[1] In₂O₃:ZnO=1:2 [mol] (Si=0 [mol %]);

[2] In₂O₃:ZnO:SiO₂=1:2:0.2 [mol] (Si=2 [mol %]);

[3] In₂O₃:ZnO:SiO₂=1:2:0.4 [mol] (Si=4 [mol %]); and

[4] In₂O₃:ZnO:SiO₂=1:2:1.0 [mol] (Si=8 [mol %]).

FIG. 2 is a graph showing results of comparison between the Si contentin an In—Si—Zn—O target and the Si content in an In—Si—Zn—O film. Inthis graph, the horizontal axis represents the Si content (mol %) in thetarget, and the vertical axis represents the Si content (mol %) in thefilm. This graph shows that the Si content in the target issubstantially equal to the Si content in the film.

The Si content in the target shown in the graph of FIG. 2 was obtainedby calculation. In addition, the Si content in the film was measured byRutherford backscattering spectrometry (RBS). The values of the Sicontents are shown in Table 1. In this specification, the Si content isexpressed as Si=0, 2, 4, or 8 [mol %] just for simplicity.

TABLE 1 Si content in target [mol %] Si content in film [mol %] 0 1.22.1 2.2 3.9 3.9 8.3 8.8

FIG. 3 is a graph showing results of measuring an In—Si—Zn—O film by anX-ray diffraction (XRD) analysis method. In this graph, the horizontalaxis represents the irradiation angle of an X ray, and the vertical axisrepresents the intensity of a peak. This graph shows that as the Sicontent in the film is increased, the intensity of a broad peak at 30deg. to 35 deg. due to In—Zn—O is weakened.

FIG. 4 is a graph showing dependence of Hall effect mobility of anIn—Si—Zn—O film on the Si content in the film. In this graph, thehorizontal axis represents the Si content in the film, the left verticalaxis represents the Hall effect mobility, and the right vertical axisrepresents the carrier density. This graph shows that as the Si contentin the film is increased, the Hall effect mobility (indicated by acircle in the graph) and the carrier density (indicated by a cross inthe graph) are decreased.

The graph of FIG. 4 shows the following results: when the Si content is4 mol %, the carrier density is lower than or equal to 1×10²⁰/cm³;similarly, when the Si content is 4 mol %, the Hall effect mobility islower than or equal to 20 cm²/Vs.

From the graphs of FIG. 3 and FIG. 4, it is found that inclusion of Sienables control of the carrier density of the oxide semiconductor film50.

Note that a sample used in the measurement for obtaining the resultsshown in the graphs of FIG. 3 and FIG. 4 is a 150-nm-thick In—Si—Zn—Ofilm which has been subjected to heat treatment at 450° C. for 1 hour inan N₂ atmosphere.

<Initial Characteristics of Field Effect Transistor Including In—Si—Zn—OFilm>

FIG. 5, FIG. 6, FIG. 7, FIG. 8, FIG. 9, FIG. 10, FIG. 11, FIG. 12, FIG.13, FIG. 14, FIG. 15, and FIG. 16 are graphs showing I_(d)-V_(g)characteristics [log(I_(d))-V_(g)] of the field effect transistorillustrated in FIG. 1. In each of these graphs, the horizontal axisrepresents the level of gate voltage V_(g) [V], the left vertical axisrepresents the amount of drain current I_(d) [A] (indicated by a solidline in the graphs), and the right vertical axis represents the fieldeffect mobility μFE [cm²/Vs] (indicated by a dashed line in the graphs).Note that the I_(d)-V_(g) characteristics were measured under thecondition that the level of drain voltage V_(d) [V] was 1 V or 10 V anda gate voltage V_(g) [V] of −30 V to 30 V was applied.

Note that in a field effect transistor used for the measurement, a100-nm-thick SiON film was used as the gate insulating film 40 and a100-nm-thick Ti film was used as the metal film 60. In addition, thethickness of the oxide semiconductor film 50 was 20 nm, the channellength L was 10 μm, and the channel width W was 50 μm. The oxidesemiconductor film 50 of the field effect transistor was formed in thefollowing manner: a substrate at room temperature was subjected tosputtering using a DC power source of 100 W in an atmosphere where theratio of gasses was Ar/O₂=67/33[%] and the total pressure was 0.4 Pa.

FIG. 5, FIG. 6, and FIG. 7 are graphs showing I_(d)-V_(g)characteristics of field effect transistors in which the Si content inthe oxide semiconductor film 50 is [1] Si=0 [mol %]. These three graphsshow results of measuring I_(d)-V_(g) characteristics of field effecttransistors which were subjected to heat treatment under differentconditions after the formation of the oxide semiconductor film 50. Thefield effect transistor of FIG. 5 was subjected to heat treatment at350° C. for 1 hour in an N₂ atmosphere. The field effect transistor ofFIG. 6 was subjected to heat treatment at 450° C. for 1 hour in an N₂atmosphere. The field effect transistor of FIG. 7 was subjected to heattreatment at 600° C. for 1 hour in an N₂ atmosphere, and then subjectedto heat treatment at 450° C. for 1 hour in an atmosphere of N₂:O₂=4:1(hereinafter this atmosphere is also referred to as dry air).

FIG. 5 and FIG. 6 show that the off-state current of each of these fieldeffect transistors is 1×10⁻¹³ A or less, the on-state current thereof is1×10⁻⁵ A or more, and the on/off ratio thereof is 10⁸ or more; thus,excellent switching characteristics are obtained. Further, the fieldeffect mobility μFE reaches 45 cm²/Vs.

However, FIG. 7 shows that the field effect transistor does not have asufficient on/off ratio. Further, the field effect transistor isnormally on.

FIG. 8, FIG. 9, and FIG. 10 are graphs showing I_(d)-V_(g)characteristics of field effect transistors in which Si content in theoxide semiconductor film 50 is [2] Si=2 [mol %]. These three graphs showresults of measuring I_(d)-V_(g) characteristics of field effecttransistors which were subjected to heat treatment under differentconditions after the formation of the oxide semiconductor film 50. Thefield effect transistor of FIG. 8 was subjected to heat treatment at350° C. for 1 hour in an N₂ atmosphere. The field effect transistor ofFIG. 9 was subjected to heat treatment at 450° C. for 1 hour in an N₂atmosphere. The field effect transistor of FIG. 10 was subjected to heattreatment at 600° C. for 1 hour in an N₂ atmosphere, and then subjectedto heat treatment at 450° C. for 1 hour in an atmosphere of N₂:O₂=4:1(dry air).

FIG. 8 and FIG. 9 show that the off-state current of each of these fieldeffect transistors is 1×10⁻¹³ A or less, the on-state current thereof is1×10⁻⁵ A or more, and the on/off ratio thereof is 10⁸ or more; thus,excellent switching characteristics are obtained. Further, the fieldeffect mobility μFE reaches 22 cm²/Vs.

However, FIG. 10 shows that the field effect transistor does not have asufficient on/off ratio. Further, the field effect transistor isnormally on.

FIG. 11, FIG. 12, and FIG. 13 are graphs showing I_(d)-V_(g)characteristics of field effect transistors in which Si content in theoxide semiconductor film 50 is [3] Si=4 [mol %]. These three graphs showresults of measuring I_(d)-V_(g) characteristics of field effecttransistors which were subjected to heat treatment under differentconditions after the formation of the oxide semiconductor film 50. Thefield effect transistor of FIG. 11 was subjected to heat treatment at350° C. for 1 hour in an N₂ atmosphere. The field effect transistor ofFIG. 12 was subjected to heat treatment at 450° C. for 1 hour in an N₂atmosphere. The field effect transistor of FIG. 13 was subjected to heattreatment at 600° C. for 1 hour in an N₂ atmosphere, and then subjectedto heat treatment at 450° C. for 1 hour in an atmosphere of N₂:O₂=4:1(dry air).

FIG. 11, FIG. 12, and FIG. 13 show that the off-state current of each ofthese field effect transistors is 1×10⁻¹³ A or less, the on-statecurrent thereof is 1×10⁻⁵ A or more, and the on/off ratio thereof is 10⁸or more; thus, excellent switching characteristics are obtained.Further, the field effect mobility μFE reaches 10 cm²/Vs.

FIG. 14, FIG. 15, and FIG. 16 are graphs showing I_(d)-V_(g)characteristics of field effect transistors in which Si content in theoxide semiconductor film 50 is [4] Si=8 [mol %]. These three graphs showresults of measuring I_(d)-V_(g) characteristics of field effecttransistors which were subjected to heat treatment under differentconditions after the formation of the oxide semiconductor film 50. Thefield effect transistor of FIG. 14 was subjected to heat treatment at350° C. for 1 hour in an N₂ atmosphere. The field effect transistor ofFIG. 15 was subjected to heat treatment at 450° C. for 1 hour in an N₂atmosphere. The field effect transistor of FIG. 16 was subjected to heattreatment at 600° C. for 1 hour in an N₂ atmosphere, and then subjectedto heat treatment at 450° C. for 1 hour in an atmosphere of N₂:O₂=4:1(dry air).

FIG. 14, FIG. 15, and FIG. 16 show that the off-state current of each ofthese field effect transistors is 1×10⁻¹³ A or less, the on-statecurrent thereof is 1×10⁻⁶ A or more, and the on/off ratio thereof is 10⁷or more; thus, excellent switching characteristics are obtained.However, the field effect mobility μFE is very low.

These graphs show that as the Si content is increased, the field effectmobility of the transistor is decreased. On the other hand, it is foundthat in the case where Si is not included or the Si content is small,the threshold voltage is decreased and the transistor is normally on asthe temperature of heat treatment performed after the formation of theoxide semiconductor film 50 is raised.

<Results of BT Test Performed on Field Effect Transistor IncludingIn—Si—Zn—O Film>

FIG. 17, FIG. 18, FIG. 19, FIG. 20, FIG. 21, FIG. 22, FIG. 23, and FIG.24 are graphs showing results of bias-temperature (BT) tests performedon the transistor illustrated in FIG. 1. In each of these graphs, thehorizontal axis represents the level of gate voltage V_(g) [V], the leftvertical axis represents the amount of drain current I_(d) [A] (in thegraphs, a heavy solid line shows results before the test and a heavydashed line shows results after the test), and the right vertical axisrepresents the field effect mobility μFE [cm²/Vs] (in the graphs, asolid line shows results before the test and a dashed line shows resultsafter the test).

The field effect transistor used for the measurement was manufactured inthe following manner. A SiON film was formed to a thickness of 100 nm asthe gate insulating film 40, and then the oxide semiconductor film 50 isformed to a thickness of 20 nm. After that, heat treatment was performedat 350° C. for 1 hour in an atmosphere of N₂:O₂=4:1 (dry air). A100-nm-thick Ti film was formed as the metal film 60, and then heattreatment was performed at 250° C. for 1 hour in an N₂ atmosphere. Inaddition, the channel length L of the field effect transistor was 20 μmand the channel width W thereof was 20 μm.

The BT tests was performed under the condition that a gate voltage of 20V (+BT) or a gate voltage of −20 V (−BT) was applied at 150° C. for 1hour. Note that in the BT test, the level of drain voltage V_(d) [V] wasset at 1 V or 10 V.

FIG. 17 and FIG. 18 are graphs showing results of the test performed ona field effect transistor in which the Si content in the oxidesemiconductor film 50 is [1] Si=0 [mol %]. FIG. 17 shows results of a+BT test, and the amount of shift in the threshold voltage is 2.66 V.FIG. 18 shows results of a −BT test, and the amount of shift in thethreshold voltage is −3.42 V.

FIG. 19 and FIG. 20 are graphs showing results of the test performed ona field effect transistor in which the Si content in the oxidesemiconductor film 50 is [2] Si=2 [mol %]. FIG. 19 shows results of a+BT test, and the amount of shift in the threshold voltage is 2.90 V.FIG. 20 shows results of a −BT test, and the amount of shift in thethreshold voltage is −2.59 V.

FIG. 21 and FIG. 22 are graphs showing results of the test performed ona field effect transistor in which the Si content in the oxidesemiconductor film 50 is [3] Si=4 [mol %]. FIG. 21 shows results of a+BT test, and the amount of shift in the threshold voltage is 6.04 V.FIG. 22 shows results of a −BT test, and the amount of shift in thethreshold voltage is −0.22 V.

FIG. 23 and FIG. 24 are graphs showing results of the test performed ona field effect transistor in which the Si content in the oxidesemiconductor film 50 is [4] Si=8 [mol %]. FIG. 23 shows results of a+BT test, and the amount of shift in the threshold voltage is 14.48 V.FIG. 24 shows results of a −BT test, and the amount of shift in thethreshold voltage is −0.12 V.

These graphs show that as the Si content is increased, the amount ofshift in the threshold voltage due to the +BT test is increased whilethe amount of shift in the threshold voltage due to the −BT test isdecreased. Accordingly, it is effective for an element to which −BTstress is always applied to include Si. Note that in the case where theSi content is small, a significant effect on improvement in the shift ofthe threshold voltage due to the −BT test is not obtained.

Consequently, with the use of an In—Si—Zn—O film in which the Si contentis greater than or equal to 4 mol % and less than or equal to 8 mol %, afield effect transistor which can withstand heat treatment at a hightemperature and is effective against −BT stress can be manufactured.

This application is based on Japanese Patent Application serial no.2009-281408 filed with Japan Patent Office on Dec. 11, 2009, the entirecontents of which are hereby incorporated by reference.

Reference Numerals

-   10: substrate, 20: base insulating film, 30: gate electrode, 35:    conductive film, 40: gate insulating film, 50: oxide semiconductor    film, 55: oxide semiconductor film, 60: metal film, 65: metal film.

The invention claimed is:
 1. A field effect transistor comprising: asubstrate; a gate electrode over the substrate; a gate insulating filmover the gate electrode; an oxide semiconductor film over the gateinsulating film; and a source electrode and a drain electrode over theoxide semiconductor film, wherein the oxide semiconductor film comprisesindium and zinc, wherein the oxide semiconductor film includes a tapershaped depression of a first taper angle between the source electrodeand the drain electrode, wherein each of the source electrode and thedrain electrode includes a taper shaped edge of a second taper angle,and wherein the first taper angle is different from the second taperangle.
 2. The field effect transistor according to claim 1, furthercomprising an insulating film between the substrate and the gateelectrode.
 3. The field effect transistor according to claim 1, aconcentration of hydrogen in the oxide semiconductor film is 5×10¹⁹atoms/cm³ or lower.
 4. The field effect transistor according to claim 1,wherein the oxide semiconductor film comprises silicon.
 5. The fieldeffect transistor according to claim 4, wherein a content of silicon inthe oxide semiconductor film is greater than or equal to 4 mol% and lessthan or equal to 8 mol%.
 6. A field effect transistor comprising: asubstrate; a gate electrode over the substrate; a gate insulating filmover the gate electrode; an oxide semiconductor film over the gateinsulating film; and; a source electrode and a drain electrode over theoxide semiconductor film, wherein the oxide semiconductor film comprisesindium and zinc, wherein an electron carrier density of the oxidesemiconductor film is 1×10²⁰ /cm³ or lower, wherein the oxidesemiconductor film includes a taper shaped depression of a first taperangle between the source electrode and the drain electrode, wherein eachof the source electrode and the drain electrode includes a taper shapededge of a second taper angle, and wherein the first taper angle isdifferent from the second taper angle.
 7. The field effect transistoraccording to claim 6, further comprising an insulating film between thesubstrate and the gate electrode.
 8. The field effect transistoraccording to claim 6, a concentration of hydrogen in the oxidesemiconductor film is 5×10¹⁹ atoms/cm³ or lower.
 9. The field effecttransistor according to claim 6, wherein the oxide semiconductor filmcomprises silicon.
 10. A field effect transistor comprising: asubstrate; a gate electrode over the substrate; a gate insulating filmover the gate electrode; an oxide semiconductor film over the gateinsulating film; and a source electrode and a drain electrode over theoxide semiconductor film, wherein the oxide semiconductor film comprisesindium and zinc, wherein Hall effect mobility of the oxide semiconductorfilm is 20 cm²/Vs or lower, wherein the oxide semiconductor filmincludes a taper shaped depression of a first taper angle between thesource electrode and the drain electrode, wherein each of the sourceelectrode and the drain electrode includes a taper shaped edge of asecond taper angle, and wherein the first taper angle is different fromthe second taper angle.
 11. The field effect transistor according toclaim 10, further comprising an insulating film between the substrateand the gate electrode.
 12. The field effect transistor according toclaim 10, a concentration of hydrogen in the oxide semiconductor film is5×10¹⁹ atoms/cm³ or lower.
 13. The field effect transistor according toclaim 10, wherein the oxide semiconductor film comprises silicon.